Managing farmed closed depressional areas using blind inlets to minimize phosphorus and nitrogen losses

Closed depressions are sites within a landscape from glacial origin in which runoff water tends to collect because there is no natural outlet. When farmed, this water is often drained via a tile riser, a vertical tube that connects the bottom of the low point of a closed depression with the subsurface tile drainage network. Two field-sized closed depressions (ca. 4ha) and two small catchments (ca. 300ha) were used to test the hypothesis that water quality could be improved by replacing tile risers with blind inlets, ca. 18m2 holes filled to 1m depth so that a high infiltration capacity was attained. At the field scale discharge, sediment and nutrient loads were lower when drained with the blind inlet compared to the tile riser. Late Spring 2010 was much wetter than normal, and increased nutrient loading was observed in both small catchments compared with previous years. In 2010, discharge and total P loading from Catchment AME (control catchment drained with tile risers) increased 417% and 737%, respectively, compared to the mean for the six previous years, whereas in Catchment BME (treatment catchment drained with blind inlets), there was only a 64% and 92% increase, respectively. Results from this study indicate that replacing tile risers with blind inlets to drain closed depressions may be one method to effectively reduce nutrient and sediment loading to Lake Erie and other sensitive water bodies that lie in landscapes formed by Wisconsin, Weichselian or Devensian glaciation.